The present invention relates to the field of amplifiers and more particularly, power amplifiers for wireless telecommunications.
There are currently many different wireless communications systems promulgated by the telecommunications industries and used in the world. These systems are complex and they set forth specifications regarding all aspects of wireless communications, including physical characteristics of signal transmission, such as transmission frequency and operation mode.
One of the earliest wireless communications systems developed in North America is called the advanced mobile phone service (xe2x80x9cAMPSxe2x80x9d). Used for analog cellular communications, AMPS specifies a mobile station transmission frequency band between 824 MHz and 849 MHz. This band is often referred to as the 800 MHz band or the cellular band. Within the same frequency band also operates a later developed system called the digital mobile phone service (xe2x80x9cDMPSxe2x80x9d), which is used for both digital and analog communications. These systems are generally referred to in the industry as AMPS 800 and DMPS 800.
A European wireless communications system, the global system for mobile communications (xe2x80x9cGSMxe2x80x9d), specifies a mobile station transmission frequency band between 890 MHz to 915 MHz and it is used for digital communications. This system is often referred to as GSM 900. Although not widely adopted in North America, GSM 900 is highly popular in Europe and parts of Asia. Recently, a new system called personal communications system (xe2x80x9cPCSxe2x80x9d) 1900, which specifies a mobile station transmission frequency between 1850 MHz and 1910 MHz, is proposed for use in North America. The transmission frequency of PCS 1900 is substantially higher than that of AMPS 800 or GSM 800.
There are many other systems. For example, the nordic mobile telephone 450 system (xe2x80x9cNMT-450xe2x80x9d) specifies a transmission frequency between 463 MHz and 468 MHz and the signal modulation technique of FDMA. The nordic mobile telephone 900 system (xe2x80x9cNMY-900xe2x80x9d) specifies a transmission frequency between 935 MHz and 960 MHz and the same signal modulation technique.
As for digital cordless telephones, there are, for example, cordless telephone 2 (xe2x80x9cCT2xe2x80x9d) requiring a transmission frequency between 864 MHz and 868 MHz and modulation technique of TDMAIFDM, and digital European cordless telephone (xe2x80x9cDECTxe2x80x9d) specifying a transmission frequency between 1886 MHz and 1990 MHz with the same modulation technique.
Those different transmission frequency bands and operating modes present a unique challenge for wireless service providers and particularly for manufactures of wireless communications equipment. If a service provider wishes to replace its currently used wireless system with one operating in a higher frequency band (e.g., from AMPS 800 to PCS 1900), the existing base stations must be upgraded so that they operate in accordance with the new system. By using upconverters which convert a lower frequency signal to a higher frequency signal, the base stations can be upgraded to operate at a higher frequency. Of course the base stations must also be updated to comply with other aspects of the new wireless system.
In addition to upgrading the base stations, individual cellular telephones in the hands of customers must also be upgraded or replaced so that they be compatible with the new wireless system. In particular, since the power amplifier used in each cellular phone is optimized to operate within a particular frequency band and at a particular mode, it needs to be replaced with a new power amplifier suitable for operation under the new wireless standard.
For example, cellular phones used for AMPS 800 contain a power amplifier optimized to operate within the cellular band (i.e., the 800 MHz band). If, however, AMPS 800 is replaced with PCS 1900, the old AMPS phones cannot be used any more; they must be upgraded or replaced. Replacing cellular phones is expensive. A new cellular phone which can be easily upgraded is desired.
For cellular phone manufactures, different wireless systems requires different power amplifiers which increases cost. It is desired that a single amplifier be used for different systems. Different wireless systems present another problem: If a cellular phone user crosses from one area served by one wireless system into an area served by a different wireless system, he will not be able to use his phone. It is desired that the same cellular phone be used under different wireless systems and that the user can simply activate a switch to use it under a different wireless system. Preferably, when a user enters into an area served by a different wireless system, the user""s phone is automatically switched to operate under the new wireless system that covers the area. This can be achieved by a base stations sending a signal to the cellular phone to switch the cellular phone. In any event, it requires a power amplifier capable of operating under different wireless systems.
U.S. Pat. No. 5,060,294 assigned to Motorola Inc. describes a dual mode power amplifier operable in either linear or saturation mode. The mode selection is accomplished with the use of a processor by (1) altering the dc bias to a power transistor in the amplifier and/or (2) altering the ac load of the amplifier to change the load line. Although the amplifier may operate in either linear or saturation mode, it is not suitable for operation at different wireless frequencies. For example, the amplifier is not suitable to operate in both the cellular band (the 800 MHz band) and the new PCS band (the 1900 MHz band).
U.S. Pat. No. 5,438,684, also assigned to Motorola Inc., describes a dual-mode RF signal power amplifier comprising two amplifying branches connected in parallel, one for non-linear mode operation such as the FM mode and the other for linear mode operation such as the TDMA digital mode. A PIN diode is connected in series with one of the branch for decoupling it from the other branch. When operating, the selected branch is turned on whereas the non-selected branch is turned off. This dual-mode power amplifier is only suitable for operation at one frequency such as 800 MHz or 1900 MHz, but not at both frequencies.
It is therefore an object of the present invention to provide a multi-band amplifier that can operate under different wireless systems and provide required power and efficiency.
The present invention provides an amplifying apparatus to operate at different frequencies or different frequency bands (e.g., the cellular band and the PCS band) and in different modes (e.g., A, B, AB or C). The amplifier can be used in cellular phones to operate under different wireless systems.
In one embodiment, the amplifying apparatus comprises a plurality of amplifiers each suitable to operate at one of a plurality of predetermined frequencies, and a control circuit. According to the frequency of input signal, the control circuit, responsive to a control signal, selectively enables the amplifier suitable for operating at the input signal frequency while it prevents the other amplifiers from operation. The control signal may be generated manually with the use of a switch or automatically by a detecting circuit which detects the frequency of the input signal; it may also be provided or triggered by a base station for wireless communications.
In this embodiment, each amplifier comprises at least one amplifying stage for amplifying the input signal. Each amplifier has input impedance means for providing predetermined input impedance and output impedance means for providing predetermined output impedance at the frequency the amplifier is suitable to operate. Preferably, the input impedance approximately matches source impedance of input signal.
In a preferred embodiment, each amplifier comprises a plurality of amplifying stages arranged as a cascade. Predetermined interstage impedance between any two successive amplifying stages is provided by interstage impedance means at the signal frequency the amplifier is suitable to operate. Preferably, each amplifier stage includes at least one amplifying transistor, and the amplifier is enabled or disabled by the control circuit by turning the amplifying transistor(s) in the amplifier on or off. The control circuit also operates to bias the selected amplifier to operate in a desired operating mode. More preferably, the amplifying apparatus comprising the amplifiers and the control circuit is a monolithic GaAs integrated circuit (xe2x80x9cGaAs MMICxe2x80x9d).
In accordance with another embodiment, an amplifying apparatus is provided with at least one amplifying stage, and input impedance means for providing, in accordance with the frequency of input signal, predetermined input impedance at the frequency of the input signal. Preferably, such input impedance matches source impedance. More preferably, the amplifying apparatus further includes output impedance means for providing predetermined output impedance at the signal frequency. Still more preferably, a bias control circuit is provided for selectively biasing the amplifying stage to operate in one of a plurality of predetermined operating modes.
In a preferred embodiment, the amplifying apparatus includes a plurality of amplifying stages arranged as a cascade. The apparatus is provided with input impedance switching means which selectively provides, in accordance with input signal frequency, one of a plurality of input impedance networks to input of a first amplifying stage. Operating with the first stage, the selected input impedance network provides predetermined input impedance which preferably approximately matches source impedance at the input signal frequency. Interstage impedance means are also provided for providing predetermined interstage impedance. More specifically, the interstage impedance means comprise means for controlling, in accordance with the input signal frequency, the impedance of an impedance network connected between a preceding stage and a dc power supply.
The preferred embodiment is further provided with output impedance switching means which selectively provides, in accordance with input signal frequency, one of a plurality of output impedance networks to output of a last amplifying stage. Operating with the last stage, the selected output impedance network provides predetermined output impedance.